Wanna downsize?
 

I ran across this intriguing engine modification on a Ducati:
Ducati V-One – Twin to Supercharged Single Conversion

Really, it should work on any multicylinder engine. All you would need are custom ground cams for one bank of cylinders, route the exhaust to a tank and then to the intake of the other bank of cylinders. With a minimum of custom work, you could half your displacement and maintain peak power levels. I'm thinking this could be done for a net gain in mpgs.